Nanoparticle optical storage apparatus and methods of making and using same

ABSTRACT

The present invention relates in general to nanoparticles exhibiting luminescence such as photostimulated luminescence or photoluminescence and optical switching processes based upon such properties, in more particular, the use of such photostimulated luminescence exhibiting nanoparticles and switching nanoparticle for optical storage apparatuses and sensors as well as methods of making and using same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. Ser. No. 10/223,764, which isa continuation-in-part of co-pending U.S. application Ser. No.10/166,313, filed Jun. 6, 2002, entitled “UPCONVERSION LUMINESCENCEMATERIALS AND METHODS OF MAKING AND USING SAME.” This application alsoclaims priority under 35 U.S.C. § 119(e) to 1) the provisional patentapplication identified by U.S. Ser. No. 60/356,542, filed on Feb. 11,2002, entitled “REVERSIBLE OPTICAL PROCESSES AND OPTICAL STORAGE OFNANOPARTICLES;” and 2) the provisional patent application identified byU.S. Ser. No. 60/313,236, filed Aug. 17, 2001, entitled “NANOPARTICLEPHOTOSTIMULATED LUMINESCENCE BASED OPTICAL STORAGE AND SENSORS.” Theentire contents of all patent applications referenced herein are herebyexpressly incorporated herein in their entirety by reference.

STATEMENT REGARDING FEDERALLY FUNDED RESEARCH

The Government owns certain rights in and to this application pursuantto (i) a grant from the National Science Foundation Grant No.DMI-0060254, (ii) a grant from the National Science Foundation Grant No.DMI-0132030, and (iii) an Air Force Office of Scientific ResearchContract No. F49620-00-C-0058.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates in general to nanoparticles exhibitingluminescence such as photostimulated luminescence or photoluminescenceand optical switching processes based upon such properties, in moreparticular, the use of such photostimulated luminescence exhibitingnanoparticles and switching nanoparticle for optical storage apparatusesand sensors as well as methods of making and using same.

2. Background of Art

The use of nanoparticles or quantum dots exhibiting photostimulatedluminescence (“PSL”) for the storage of digital information offers atremendous advance in the digital storage art. Traditionally,cost-effective storage of information required ultrahigh packingdensities as well as inexpensive self-assembling techniques and fastmethods for writing and retrieving information. Semiconductor quantumdots (QDs), which involve a few thousand atoms, offer an attractive pathtoward achieving these goals. Single-electron storage has been suggestedin the art as a possibility with quantum dots. Charge storage devicesbased on the resistivity change of a two-dimensional electron gaslocated near a layer of self-assembled QDs have been demonstrated bythose skilled in the art at low temperature. A memory cell based onfield-effect tunable lateral potential modulation in the plane of asemiconductor quantum well and acoustically driven storage in quantumwells have also been proposed and demonstrated by those skilled in theart. The storage characteristics of these approaches are limited toshort duration (several hundred microseconds) at very low temperatures.

Optical storage and retrieval of excitons in semiconductorself-assembled quantum dots have been demonstrated but the storage timeswere only on the order of several seconds at very low temperatures. Sucha short storage time is not long enough for practical application.Herein is claimed and disclosed an optical storage apparatus and methodsof use utilizing nanoparticles exhibiting photostimulated luminescenceand/or optical switch process at room temperature with a storagelongevity on the order of multiple hours.

Photostimulated luminescence is a process in which trapped charges arereleased by photons to produce luminescence through recombination andhas been previously studied extensively in the area of X-ray radiology.The use of photostimulable BaFBr:Eu²⁺ phosphors for X-ray storage andimaging has proven to be one of the most successful detectors in digitalradiography. The photostimulated luminescence mechanism of BaFBr:Eu²⁺involving X-ray irradiation proposed by Takahashi et al. assumed that,during X-ray irradiation, the Eu²⁺ ions are partly ionized into theirtrivalent charge state (Eu³⁺) and the liberated electrons drift via theconduction band to form F centers. Upon subsequent photostimulation, theelectrons are released from the F centers into the conduction band andthereafter recombine with Eu³⁺ ions to produce the photostimulatedluminescence of Eu²⁺ at 390 nm.

A general erasable optical storage apparatus using photostimulatedluminescence is shown displayed in FIG. 1. As shown in FIG. 1, thewriting light can be either ultra-violet (UV) or blue or any other lighthaving energy higher than the energy gap of the host materials (i.e. thewriting light is variable and will depend on the energy gap of the hostmaterial). The reading light can be visible or infrared (IR) light, thechoice of reading light is also variable and depends on the trap depthof the host material. Semiconductors such as MgS, CaS, SrS, and SrSedoped with rare earth elements such as Ce, Sm, and Eu have beenpreviously considered for optical storage and dosimetric applications.These materials possess a high sensitivity for radiation energy storageand a high PSL efficiency under IR stimulation. However, they sufferfrom significant fading at room temperature due to their narrow bandgaps and the fact that their shallow trap depths do not efficientlystore energies. The PSL nanoparticles of the presently claimed anddisclosed invention overcome such limitations in the art.

An application of PSL phosphors is medical imaging storage. Such PSLphosphors must posses the properties of high density, high brightness,short decay lifetimes, suitable emission energy and stimulation energyand low light scattering. The energy structure in the phosphors iscritical to the effective operation of the detector. The trap depth tothe conduction band must be small enough so that stimulation with laserlight is possible, yet sufficiently large to prevent random thermalrelease of the electron from the trap. Generally, the trap depth shouldbe larger than 0.5 eV to prevent thermal release or fading at roomtemperature. In addition, the wavelength separation between thestimulation light for reading (corresponding to the color centerabsorption band) and the monitored emitting light of the recombinationcenter should be sufficiently large so that noise signals due to readinglight reflection are avoided. The phosphors may have only one type oftrap to thereby reduce signal loss due to electron migration amongdifferent traps. Currently in the art, there is no PSL phosphor thatmeets all of these stated requirements. The photostimulable phosphorused in commercial X-ray imaging system is BaFBr:Eu²⁺. Yet, theBaFBr:Eu²⁺ system suffers from poor resolution due to the long decaylifetime of EU²⁺ emission (0.8 μs) as well as scattering of thestimulating laser light from the plate-like polycrystals. In addition,the hygroscopic nature of this phosphor limits the stability of thesystem.

Recently, it was disclosed that CdSe nanoparticles can transfer in ametastable ‘dark’ state by heating, in which the luminescence isquenched. Further, the luminescence returns when the material is exposedto light. Such a nanoparticle capable of reversible change is alsouseful for optical storage and sensor applications. Contrary to thisdisclosure, another group observed that it is possible to turn onfluorescence in thin layers of silver oxide, which are initiallynon-fluorescent, by exposing them to a light source having wavelengthsless than 520 nm. Once photoactivated, the silver oxide layers exhibitmulticolored fluorescence under excitation by both blue (450 to 480 nm)and green (510 to 550 nm) light. Blue excitation causes the layer tofluoresce in multicolored hues, but only intermittently. Greenexcitation, however, results in brighter and steadier red fluorescence.Such “caged” fluorescent nanoparticles of silver may be rapidly switchedon and used, for example, as nanoscopic optical storage elements or asprobes in living systems. Similarly, an additional group showedphotosensitivity in Ag⁺ doped phosphate. Presently, claimed anddisclosed is an optical storage apparatus(es), sensors, and methods ofmaking and using same based on the reversible optical processes ofrare-earth ion doped nanoparticles, such as Y₂O₃:Eu3+ nanoparticles, atroom temperature.

SUMMARY OF THE INVENTION

The present invention relates in general to nanoparticles exhibitingluminescence such as photostimulated luminescence or photoluminescenceand optical switching processes based upon such properties, in moreparticular, the use of such photostimulated luminescence exhibitingnanoparticles and switching nanoparticle for optical storage apparatusesand sensors as well as methods of making and using same.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is schematic illustration of PSL process in Eu²⁺ dopednanoparticles.

FIG. 2 is a schematic model of PSL storage mechanisms.

FIG. 3 is a graphical representation showing photoluminescenceexcitation (PLE, emission at 407 nm) and emission photoluminescence (PL,excitation at 304 nm) spectra of BAFBr:Eu²⁺ nanoparticles.

FIG. 4 is a graphical representation of the emission spectra ofBaFBr:Eu²⁺ nanoparticles before (1) and after UV irradiation at 254 nmfor 3 (2), 5 (3), 10 (4), and 15 (5) minutes, respectively with theexcitation at 304 nm.

FIG. 5 is a graphical representation of 304 nm excited photoluminescence(PL) and 600 nm stimulated PSL spectra of BaFBr:Eu²⁺ nanoparticles afterUV irradiation for 10 minutes, respectively.

FIG. 6 is a graphical representation of PSL decays of bulk andBaFBr:Eu²⁺ nanoparticles excited at 560 nm.

FIG. 7 is a graphical representation showing fluorescence excitationspectra of AgI/Y before (a) and after UV irradiation at 254 nm for 5(b), 8 (c), 11 (d), and 15 minutes (e) respectively.

FIG. 8 is a graphical representation showing photoluminescence spectraof AgI/Y before (a) and after (b) UV irradiation at 254 nm for 5minutes. After exposure to a visible lamp for 5 minutes (c).

FIG. 9 is a graphical representation showing PSL spectra of AgI/Y afterUV irradiation at 254 nm for 10 minutes and thereafter excitation at 840nm.

FIG. 10 is a graphical representation showing electron spin resonance ofAgI/Y before (a) and after UV irradiation at 254 nm for 8 minutes (b)and 15 minutes (c), respectively.

FIG. 11 is a graphical representation showing the 310 nm excitedphotoluminescence spectra of Ag/Y before (a), and after UV irradiationat 254 nm for 10 minutes (b), then exposure to a 840 nm light for 10minutes (c), then a 650 nm light for 10 minutes (d), and then to avisible lamp for 10 minutes (e). Trace (f) is the photostimulatedluminescence spectrum stimulated at 840 nm.

FIG. 12 is a graphical representation showing lifetime decays of (a)photoluminescence (excitation at 305 nm) and (b) photostimulatedluminescence (excitation at 800 nm) in AgI/Y nanoparticles. The inset in(a) displays the PL lifetime at higher time resolution showing the fast(<15 ns) time component.

FIG. 13 is a graphical representation showing lifetime decays of (a)photoluminescence (excitation at 305 nm) and (b) photostimulatedluminescence (excitation at 800 nm) in Ag/Y nanoparticles. The inset in(a) displays the PL lifetime at higher time resolution showing the fast(<15 ns) time component.

FIG. 14 is a schematic representation of the energy levels of thephotophysical processes in Ag/Y and AgI/Y nanoparticles. Solid linesrepresent photon excitation, dashed lines represent luminescence, anddotted lines represent energy transfer between levels.

FIG. 15 is a graphical representation showing the emission spectra ofBaFBr:Eu³⁺ nanoparticles before (1) and after UV irradiation at 225 nmfor 5 (2), 10 (3) and 15 min (4), then exposure to a 600 nm light for 5min (5) and then to a visible lamp for 15 min, respectively.

FIG. 16 is a graphical representation showing the emission spectra ofEu³⁺-zeolite-Y before (1) and after (2) UV irradiation at 254 nm for 5min, and then to a visible lamp for 10 min (4), respectively.

FIG. 17 is a graphical representation of how electrons and holes areproduced in nanoparticles and, more specifically, in Eu³⁺ dopednanoparticles.

FIG. 18 is a graphical representation of the emission spectra ofY₂O₃:Tb³⁺, Eu³⁺ nanoparticles before and after x-ray irradiation for 10minutes.

FIG. 19 is a graphical representation of the absorption(a),fluorescence(b) and PSL(c) of CdTe nanoparticles.

FIG. 20 is an X-ray diffraction pattern of ZnS:Mn²⁺,Eu³⁺ nanoparticles.

FIG. 21 is an HRTEM image of ZnS:Mn²⁺,Eu³⁺ nanoparticles.

FIG. 22 is an emission spectra of ZnS:Mn²⁺,Eu³⁺ nanoparticles.

FIG. 23. graphically depicts the photosensitivity of ZnS:Mn²⁺,Eu³⁺nanoparticles.

FIG. 24 graphically depicts the photosensitivity of ZnS:Mn²⁺,Eu³⁺nanoparticles.

FIG. 25 is a schematic illustration ZnS:Mn²⁺,Eu³⁺ nanoparticles foroptical storage.

FIG. 26 is a a schematic illustration of a digital imaging system foroptical digital imaging and diagnostics.

FIG. 27 is a side-elevational view of a nanoparticle thin film imagingplate.

FIG. 28 is a schematic illustration of a storage device constructed inaccordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Before explaining at least one embodiment of the invention in detail, itis to be understood that the invention is not limited in its applicationto the details of construction, experiments, exemplary data, and/or thearrangement of the components set forth in the following description orillustrated in the drawings. The invention is capable of otherembodiments or of being practiced or carried out in various ways. Also,it is to be understood that the phraseology and terminology employedherein is for purpose of description and should not be regarded aslimiting.

Upon activation by light, target nanoparticles are capable of beingchanged back and forth from one state to the other. One such example isshown in FIG. 1 for Eu²⁺-doped nanoparticles. One type of state changeis accompanied by a luminescence in which the excitation energy is lowerthan the emission energy. This type of luminescence is calledphotostimulated luminescence (PSL). The mechanism of PSL storage isillustrated in FIG. 2. In a doped nanoparticle such as ZnS:Eu²⁺, uponexcitation or irradiation of the particles with an energy gap largerthan the energy gap of the host (i.e. ZnS), electrons and holes aregenerated in the conduction and the valence bands, respectively.Meanwhile, defects are also created. Electrons may be trapped at thedefects to form color centers, and holes may combine with Eu²⁺ to formEu³⁺ or (Eu²⁺+h) complex. When the nanoparticles are stimulated withlight corresponding to the absorption band of the color centers, theelectrons will be detrapped from the defects and recombine with Eu³⁺ or[Eu²⁺+h] to give the luminescence of Eu²⁺ which is called PSL.

The concept of PSL is herein used to develop a general erasable opticalstorage material and apparatus developed therefrom as displayed in FIG.2. For use with a generable erasable optical storage material, thewriting light can be ultra-violet (UV, including vacuum-UV andextreme-UV),blue, X-ray, α-particle, β-particle or γ-ray as determinedby the energy gap of the host materials. The reading light can bevisible or infrared (IR) as necessitated by the trap depth.Semiconductors like MgS, CaS, SrS, and SrSe, doped with different rareearth elements such as Ce, Sm, and Eu, have been considered for opticalstorage and dosimetric applications. These materials possess a highsensitivity for radiation energy storage and a high PSL efficiency underIR stimulation. However, they suffer from significant fading at roomtemperature due to their narrow band gaps and the shallow traps forstoring the energy.

In nanoparticles, the energy structure can be modified via quantum sizeconfinement. This offers a new way of designing PSL phosphors. Whenelectrons and holes are produced in nanoparticles by excitation, theelectrons and holes may de-excite or relax to the lowest excited statesand recombine to give luminescence. They also may be trapped by electronor hole traps at the surfaces, interfaces, or/and in the surroundingmatrix. The electrons or holes at traps are in a metastable state. Whenstimulated by light or by heat some electrons or holes may be releasedand go back to the nanoparticles, recombining to provideluminescence—i.e., photostimulated luminescence (PSL) orthermoluminescence.

Controlled charge separation and trapping in nanoparticles are key tothe instantly described and claimed materials, apparatus(es), andprocess(es) for reversable optical storage, sensors, and digitalimaging. The PSL of nanoparticles requires not only charge separationand trapping, but also the return of the carriers to the nanoparticles.Evidence for the charge separating, trapping, and returning has beenobtained from pump-probe measurements and photon-gated hole-burning(PHB) of nanoparticles. For example, in BaFCl:Sm²⁺, the hole-burningprocess can be described as the photoionization reaction:Sm²⁺+(trap)⇄Sm³⁺+(trap)⁻.

If the electrons release from the traps and return to the Sm²⁺, the holewill be erased and is called hole-filling. The hole-burning correspondsto the photoionization process in PSL and the hole-filling is similar tothe photostimulation. Thus, the occurrence of PHB in a system is anindication of PSL, and vice versa.

Hole-burning, hole-filling, and their mechanisms in nanoparticles havebeen investigated by several groups. The photo-excited exciton in ananoparticle is localized and an electron or hole is trapped at thesurface of the nanoparticle. A hole or electron escapes from thenanoparticle, tunnels through the potential barrier and is trapped inthe host, referring to the hole-burning process. By thermal annealing orlight stimulation, the electron or hole releases and returns to thenanoparticles, essentially serving as a hole-filling. When the carriersreturn to the nanoparticles and recombine to give luminescence, theyproduce photostimulated luminescence.

In most systems, hole-burning is only observable and stable at lowtemperatures. Only in a few materials is hole-burning at roomtemperature possible. However, PSL is not dependent on temperature. Forexample, in a BaFCl_(x)Br_(1-x):Sm²⁺ system (where x>0 or x<=1), lowtemperature is necessary to observe PHB, while strong PSL is observed atroom temperature. The hole-burning efficiency or the storage densityincreases by a factor of Γ_(I)/Γ_(H) (where Γ_(H i)n the inhomogeneousline width and Γ_(H) is the homogeneous line width). This ratio ishighly temperature dependent. Phonon-broadening causes Γ_(H) to increasewith temperature and makes hole-burning difficult at high temperature.Therefore, it is necessary to conduct efficient hole-burning at lowtemperature. Phonon-broadening has little effect in the PSL process.Thus, it is unnecessary to work at low temperature. This is one of theadvantages of nanoparticle PSL and forms the basic premise for theconstruction of a doped nanoparticle that exhibits PSL for use instorage devices, sensors, and optical image display systems.

Photostimulated luminescence of nanoparticles was first reported by Chenet al. (chen, Wei et al., “Photostimulated Luminescence of SilerClusters in Zeolite-Y”, Physics Letters A, vol. 232, pp. 391-394(1997)). The UV-induced PSL shows that Ag nanoclusters formed inzeolite-Y can be used for erasable optical storage. The efficient PSL ofAg clusters in zeolite-Y is closely related to the formation of thenanoclusters and the host (zeolite) properties. The Ag nanoclusters wereformed by ion-exchange in solution:nAg⁺+(Na⁺, K⁺)Z→(Ag⁺)_(n)Z+n(Na⁺,K⁺).

Then heat treatment was carried out under vacuum and electron traps wereformed as follows:(Ag⁺)_(n)+ZO^(2−→)½O₂+Ag_(n)+Z,where Z refers to the zeolite framework with a missing oxygen link. SomeLewis acid sites or oxygen vacancies are formed in the zeolite frameworkduring the heat treatment. This is a kind of electron trap. Thus, whenirradiation at 254 nm is conducted, electrons may be ionized from theclusters and trapped in the oxygen vacancies of the zeolite framework toform color centers with absorption at 840 nm:Ag_(n)→(Ag_(n))⁺+ne,e+Z→F-centers.

When the photoionized sample is stimulated at 840 nm, the trappedelectrons are released so that they return to the nanoclusters, andrecombine with (Ag_(n))⁺, producing PSL as follows:e+(Ag_(n))⁺→Ag_(n)*→Ag_(n)+540 nm

Similar results were observed in AgI nanoclusters formed in zeolite-Y byChen et al. (Chen Wei et al., “Photostimulated Luminescence of AgIClusters in Zeolite-Y”, Journal of Applied Physics, vol. 83 iss. 7, pp.3811-15 (1 Apr. 1998)). In this material, there are two kinds ofelectron traps: one in the zeolite framework and the other in the AgInanocrystal. These two types of traps do not interfere with each other.Energy can be stored by trapping electrons at these two trapssimultaneously and these levels can read separately. This, in essence,creates a “fourth dimension” for two levels of storage. Recently, PSL ofCu⁺from CuCl nanoparticles embedded in NaCl crystals was observed attemperatures lower than 120° K. It was suggested by the investigatorsthat the photoionization of quantum dots and the succeeding carriercapture at the traps near the quantum dots caused the observed PSL.

The absorption and emission wavelengths of semiconductor nanoparticlesare tunable via quantum size confinement. As described hereinafter, theemission wavelength of dopants in doped nanoparticles such as ZnS:Mn,ZnS:Eu²⁺ are size dependent due to z size dependent electron-phononcoupling and the change of the crystal field with size. Electron-phononcoupling results in a size dependent Stokes-shift. As a result, theemission wavelength of doped ions is size dependent. Utilizing thisinformation and phenomena leads to the materials, apparatuses andmethodologies of the presently claimed and disclosed invention relating,in general, to design of nanoparticle materials with an emission thatfits the detector (photomultiplier) sensitivity and with absorption thatmatches available lasers. Energy levels of surface states or defects arealso size dependent, shifting to higher energies with decreasing size.Since the photoionized carriers are trapped in the surface states ordefects, the trap depth can be tuned by size effect and, thus, controlthe storage time, stability and the reading light wavelength of opticalstorage devices, sensors, and optical imaging systems.

One of the good reasons for using nanoparticles for PSL imaging is thatnanoparticles may have higher emission quantum efficiency and shorterdecay lifetime than conventional bulk phosphors. Oscillator strength ofthe semiconductors determines the absorption cross section,recombination rate, luminescence efficiency, and radiative lifetime. Theoscillator strength of the free exciton is given by:$f_{ex} = {\frac{2m}{\hslash}\Delta\quad E{\mu }^{2}{{U(0)}}^{2}}$where m is the electron mass, ΔE is the transition energy, μ is thetransition dipole moment, and |U(0)|² represents the probability offinding the electron and hole at the same site (the overlap factor). Innanostructured materials, the electron-hole overlap factor increaseslargely due to the quantum size confinement, thus yielding an increasein oscillator strength. The oscillator strength is also related to theelectron-hole exchange interaction which plays a key role in determiningthe exciton recombination rate. In bulk semiconductors, due to theextreme dislocation of the electron or hole, the electron-hole exchangeterm is very small, while in molecular-size nanoparticles, due to theconfinement, the exchange term is very large. Thus, a large enhancementof the oscillator strength from bulk to nanostructured materials isseen. Consequently, there is a large enhancement in the luminescenceefficiency.

In doped semiconductors, the excitons are bound to impurity centers. Inthis case, the oscillator strength is given by:f=f _(ex) |∫dxF(x)|²/Ω_(mol),

where f_(ex) is the oscillator strength of the free exciton, Ω_(mol) isthe volume of one molecule. The oscillator of a bound exciton isactually given by f_(ex) multiplied with the number of molecules coveredby the overlap of the electron and hole wave functions. Quantum sizeconfinement also enhances the bound exciton oscillator strength in dopednanoparticles.

The radiative decay lifetime(τ) is closely related to the oscillatorstrength of a transition:τ=4.5(λ_(A) ² /nf),where n is the refractive index, λ_(A) is the wavelength. The lifetimeis shortened with decreasing size due to the increase of the oscillatorstrength, f. The oscillator strength per unit volume, f/V (V is thevolume of the cluster), determines the absorption cross section. f/V isincreased upon decreasing size. Thus the absorption cross sectionincreases with decreasing size. Thereby, the absorption intensity isincreased upon decreasing size. This is good for photoionization andenhances the efficiency of the storage.

Based on the size tunability of nanoparticle properties and consideringthe requirements for PSL phosphors (Table-1), nanophase PSL phosphorswere developed (and are hereinafter described in detail) having highefficiency and fast response. In addition, the luminescenceconcentration quenching is very weak in nanoparticles. Nanoparticleshave no or very low scattering because their sizes are comparable withthe wavelength. TABLE 1 Considerations PSL Imaging PhosphorsConsiderations Purposes High Density High absorption Short luminescencelifetime Minimum scan (readout) time Low afterglow Low image noise Highefficiency and brightness Low image noise Optimum stimulation spectrumConvenient laser wavelength Large stimulation cross-section Low laserpower (High absorption oscillator strength) Low scattering High imgaeresolution Optimum fluorescence spectrum Detector sensitivity Doselinearity Signal reliability Erasure and reusability Low cost

They are easy to produce at low temperature. All of these benefits ofnanoparticle PSL phosphors indicates, and the following data confirms,that efficient PSL storage phosphors with high density and low cost,which are beneficial for practical applications, are possible and, wereindeed, produced and are enabled herein.

As described hereinafter, the photoluminescence and PSL of Ag and AgInanoclusters formed in zeolite-Y were studied using fluorescencespectroscopy. The photoluminescence spectra of AgI nanoclusters showemission from both AgI and Ag nanoclusters, although using PSL only theemission of Ag clusters is observed. While the photoluminescence fromboth Ag and AgI particles displays both sub-nanosecond and microsecondlifetimes, the emission from PSL results in very short, picosecondlifetimes. The strong PSL with short decays in nanoparticles doped withrare earth ionss and transition ions, such as Eu2+, Ce3+, Ag+, and Cu+etc., are useful in digital storage, sensor and medical radiologyapplications.

Silver and silver halide (AgX) clusters are well studied due to theirapplications in photography where the absorption of light results in theformation of small Ag clusters at the surface of the halidemicrocrystal. These clusters then catalyze the reduction of the entireAgX microcrystal during the development process. In addition, silver andsilver complexes encapsulated in zeolites are shown herein asphotocatalysts for a variety of reactions, and as a medium for opticalstorage. Thus, small clusters of these materials, such as nanoparticles,may have unique advantages for photosensitive applications. Both Ag andAgI nanoparticles encapsulated within zeolite-Y are shown herein toexhibit strong PSL.

Additionally, nanoparticles of BaFBr:Eu²⁺ exhibiting strong PSL withfaster decays than that of bulk BaFBr:Eu²⁺ are contemplated for use.Nanoparticles of ZnS:Ag⁺, ZnS:Cu⁺ and ZnS:Eu²⁺ may also be used for PSLstorage. Hereinafter, we describe their PSL and storage behaviors.

Due to quantum size confinement, the luminescence efficiency innanophase materials may be enhanced relative to bulk materials. Inaddition, the luminescence wavelength is tunable with size. Lightscattering is significantly reduced in nanoparticles compared withmicron-sized particles, since the light scattering intensity isproportional to the 6^(th) power of the particle size. Therefore,nanophase materials represent an efficient PSL phosphor for X-raystorage. The phenomenon of PSL from nanoparticles has recently beenreported by several groups. Herein claimed and described are PSLdynamics from both Ag and AgI nanoparticles encapsulated in zeolite-Y.The results show a significant reduction in the lifetime of the PSLrelative to the normal photoluminescence (PL) in both particles.

The preparation of Ag and AgI clusters in zeolite-Y (henceforth Ag/Y andAgI/Y) has been reported by Chen et al. (Chen, Wei et al.,“Photostimulated Luminescence of Silver Clusters in Zeolite-Y”, PhysicsLetters A, vol. 232, pp. 391-394 (1997)), and Chen et al. (Chen Wei etal., “Photostimulated Luminescence of AgI Clusters in Zeolite-Y”,Journal of Applied Physics, vol. 83 iss. 7, pp. 3811-15 (1 Apr. 1998U),each of which is expressly incorporated herein by reference in itsentirety and one of ordinary skill in the art, given these references,would be capable of preparing the Ag/Y and AgI/Y. In particular, Ag⁺ions were first exchanged into the cages of the zeolite. The zeolitepowder was slurried in deionized water with the acidity adjusted to pH 6with nitric acid. Silver nitrate was added and the mixture was stirredat room temperature for 2 hours. The Ag⁺ ion-exchanged zeolite wascollected by filtration and then washed with deionized water until Ag⁺was not detected in the filtrate. Ag clusters were formed in the zeolitecavities by vacuum heat treatment at 250° C. in the dark. To prepare AgIclusters, the resulting Ag⁺ ion-exchanged zeolite powder was slurried ina sodium iodide solution by stirring at 100° C. for 2 hours, after whichthe materials were collected by filtration and washed extensively withdeionized water and finally dried and calcined at 250° C. in the darkand under vacuum for 2 hours. It has been previously shown in the artthat these particles are between 1 and 2 nm in size, which correspondswell with the size of the sodalite cages of the zeolite.

The photoluminescence and PSL spectra were recorded on a SPEX FLUOROLOG™3 (obtainable from Jobin Yvon, Ltd. located in the United Kingdom)fluorescence spectrophotometer. A pulsed nanosecond optical parametricoscillator/amplifier (OPO) (Spectra-Physics MOPO-730) operating at a 10Hz repetition rate was used to collect the PL lifetime data. The outputof the OPO was frequency doubled in Potassium Dihydrogen phospate (KDP)to produce the PL excitation light. The excitation light was directedonto the particles and emission was collected and focused into a ⅛ metermonochromator equipped with a standard photomultiplier tube. Thephotomultiplier tube output was directed into a digital oscilloscope torecord the emission decays. The response time of the system was measuredto be about 15 ns FWHM.

The PSL lifetimes were too short to be measured with the nanosecond OPOsystem. In this case, excitation was provided by the output of afemtosecond regeneratively amplified titanium:sapphire laser systemoperating at 1 kHz. The 150 fs pulses of this laser at 800 nm and 200microjoules were directed onto the particles and the emission wascollected and focused into a streak camera (Hamamatsu C5680). Suitablebandpass and cutoff filters were used to collect the luminescence atdifferent wavelengths. The time resolution was determined to be about 14ns full width at half maximum (FWHM).

AgI nanoclusters encapsulated in zeolite-Y show strong luminescence andphotostimulated luminescence at room temperature. FIG. 3 displays theexcitation spectra of AgI/Y particles when monitoring an emissionwavelength corresponding to the emission maximum of Ag particles (˜510nm). The excitation peak at 265 nm is due to AgI nanoclusters, while theexcitation peak at 305 nm results from Ag nanoclusters. Under UVradiation at 254 nm, the intensity of the AgI peak decreases, while theintensity of the Ag peak increases. The AgI/Y emission intensityrecovers under subsequent illumination using a visible source. Sincesilver halides are known to produce metallic Ag clusters uponirradiation it is not surprising to find significant Ag particleluminescence from the AgI/Y particles. In addition, Ag is known toco-exist with AgI in zeolite-Y, therefore PL from Ag clusters isobserved in AgI/Y particles even in non UV-irradiated samples.

FIG. 4 shows the PL spectra of AgI/Y following excitation at 305 nm (thepeak of the Ag nanocluster absorption). The broad emission band consistsof two sub-bands: the first results from AgI nanoparticle emissionpeaking at 474 nm, and the second results from Ag nanocluster emissioncentered at 510 nm. The luminescence decreases in intensity when thesample is irradiated by ultraviolet light at 254 nm. This decrease canbe partially recovered by exposing the sample to visible light. WhenAgI/Y particles are excited at 275 nm (near the peak of the AgIabsorption), there is a noticeable red shift in the PL spectrum relativeto excitation at 305 nm.

After a few minutes of UV irradiation, strong PSL can be detected fromAgI nanoparticles in zeolite-Y as shown in FIG. 5. PSL excitation iseasily stimulated using near infrared wavelengths. FIG. 5 shows that theemission consists almost exclusively of Ag nanoclusters resulting in anarrower band than the PL emission band, and the PSL emission band isred-shifted from the PL emission band.

Additionally, after UV irradiation, an electron spin resonance signal isdetected (as shown in FIG. 6). The g-value (2.002) of this signal isclose to that of the F-center (2.0023), indicating that the signal isfrom electron centers created either in the zeolite matrix or else inthe nanoparticles themselves. These color centers are the source of theelectrons released during the PSL process.

Similarly, Ag nanoclusters in zeolite-Y are photosensitive and exhibitstrong photostimulated luminescence. FIG. 7 shows the emission spectraof Ag/Y particles following excitation at 310 nm. We observe that underUV irradiation at 254 nm for 10 minutes, the luminescence intensitydecreases significantly (FIG. 7, line “b”). The luminescence increasesin intensity slightly when the sample is then irradiated at 840 nm (FIG.7, line “c”). The luminescence increases further by exposure to a 650 nmlight for 10 minutes. (FIG. 7, line “d”). The luminescence is almostback to its original intensity after exposing to a visible lamp for anadditional 20 minutes (FIG. 7, line “e”). After UV irradiation, strongphotostimulated luminescence is observed from Ag nanoclusters, which isshown in FIG. 7, line “f”. As in AgI/Y particles, the PSL spectrum isslightly shifted to a longer wavelength from the photoluminescencespectrum of the clusters. Similar to AgI/Y, Ag/Y particles show adecrease in PL after UV irradiation. This decrease is almost completelyreversible following irradiation by light between 650 and 900 nm. Thus,both materials can be used as reusable image or digital storage media.

The PL and PSL decay lifetimes of AgI/Y are shown FIGS. 8(a-b). PL andPSL luminescence lifetimes from Ag/Y particles are shown in FIGS. 9(a-b)for comparison. The insets show PL spectra taken with higher timeresolution and show the existence of two time regimes. The first is veryfast, less than the instrument response of 15 ns. The second regime ison the order of microseconds. Both the fast and slow lifetime componentsare observed at all emission wavelengths.

The microsecond PL lifetimes and PSL lifetimes from both particles showmulti-exponential behavior at all combinations of emission andexcitation wavelengths. However there are some very clear trends whichare summarized below and in Table II:

PL from both Ag/Y and AgI/Y show both fast (<15 ns) and slow(microsecond) components at all emission wavelengths followingexcitation at either 275 nm or 305 nm.

For both Ag/Y and AgI/Y particles, the slow PL decays are noticeablyshorter at blue emission wavelengths than for red emission followingexcitation at either wavelength. In addition, the PL lifetime from Ag/Yparticles is about 3 times faster than that from AgI/Y particles. Thelonger time component ranges from about 50 to 80 microseconds in Ag/Yparticles while in AgI/Y nanoclusters, the lifetime ranges from 175 toabout 200 microseconds, depending on monitored emission wavelength.

The PSL lifetimes from both Ag and AgI show only fast components. Onecomponent is instrument limited (<14 ps) while the other varies between50 picoseconds and 800 picoseconds depending on sample and emissionwavelength. The very fast, instrument-limited luminescence may be due toextremely fast trapping of the excited electron as has been observed incolloidal AgI nanoparticles.

The PSL lifetimes are shorter for blue emission wavelengths relative tothe red emission wavelengths for both Ag and AgI particles. In addition,PSL lifetimes for Ag/Y particles are longer than the correspondinglifetimes for AgI/Y particles. For Ag/Y particles the longer componentvaries from 160 ps at 460 nm to 830 ps at 600 nm. In AgI/Y particles,this decay component ranges between 90 ps at 460 nm to about 200 ps at600 nm. TABLE II PL and PSL lifetimes of Ag/Y and AgI/Y nanoparticles.Values shown are the longest decay components of a multi-exponentialfit. Errors ± 10%. Emission Ag/Y Particles AgI/Y Particles (nm) PLLifetime^(a) PSL Lifetime^(b) PL Lifetime^(c) PSL Lifetime^(b) 460 nm 50μs 160 ps 175 μs  90 ps 500 nm 65 μs 320 ps 185 μs 125 ps 540 nm 67 μs670 ps 191 μs 170 ps 600 nm 80 μs 830 ps 208 μs 190 ps^(a)Excitation at 305 nm.^(b)Excitation at 800 nm.^(c)Excitation at 275 nm

Any model proposed to explain these results must, at a minimum, explainthe following observations: (1) Photoluminescence from Ag particles isobserved following excitation at the absorption maximum of AgI in AgI/Yparticles, (2) The observed photostimulated luminescence spectrum fromAgI/Y particles is composed almost entirely of luminescence from Agparticles, (3) Despite the fact that Ag is a metal and AgI asemiconductor, the PL and PSL lifetimes from both particles areremarkably similar, and (4) in the PL lifetime data of both particles,there are two distinct time regimes, while in the PSL lifetime data,there is only a fast component.

Shortened PSL lifetimes relative to PL lifetimes have been observedpreviously. In CaS:Eu, Sm phosphors, the PSL decay time of Eu²⁺ is lessthan 14 ps, which is much shorter than the spontaneous emission lifetimeof Eu²⁺. Similar results have been observed in SrAl₂O₄:Eu²⁺, Dy³⁺, wherea factor of three reduction in the PSL versus the PL lifetimes has beenobserved. In the above cases this lifetime shortening has been explainedas resulting from 1) fast retrapping, 2) lattice relaxation, 3) byresonant energy transfer or 4) rearrangement of the Eu²⁺ levels.

Ag photoluminescence from AgI/Y particles has been observed previously.An ‘autoreduction’ mechanism has been invoked to explain the appearanceof Ag along with AgI in these nanoclusters. That is, the zeolite acts toreduce silver cations incorporated in the zeolite cages during thenanoparticle fabrication process. In addition, irradiation of AgI isknown to produce small Ag clusters on the surface and our PL results onAgI/Y indicate that UV irradiation produces an increase in Agluminescence concurrent with a decrease in the AgI luminescence.Therefore, it appears that there are small Ag clusters likely on thesurface of the AgI particles. Excitation of AgI results in luminescencefrom Ag particles reveals that energy transfer takes place between thetwo species as has been observed previously. The close proximity of thetwo moieties would facilitate such transfer. Similar results have beenreported for nanoscale silver oxide.

FIG. 5 demonstrates that PSL from AgI/Y particles is dominated by theluminescence from Ag particles. In addition, both systems show extremelyfast PSL decay times. Although these decay times differ by about afactor of 3, they do not show the large differences that are expectedgiven that Ag is a metal while AgI is a direct bandgap semiconductor.The differences in PL and PSL lifetimes may well reflect the differentenergetics between the two, but these differences are not extremelypronounced. In addition both particles show identical PL and PSLlifetime behavior, i.e. both fast and slow components in the PL versusonly a fast component in the PSL. These observations lead to theconclusion that the PSL photophysics within AgI/Y particles is primarilydetermined by Ag clusters on the surface of the AgI nanoclusters.

The PL lifetimes from both particles show multi-exponential behaviorwith two very distinct time regimes, a fast (<15 ns) and a slow (50-100microsecond) timescale. Thus, there are two distinct subsets either ofAg or AgI particles, or environments within the zeolite sample. Thereare many possible explanations for the existence of these two timeregimes. (i) The fast decay may be a result of differences in thestrength of the transition moment of some species relative to others.This could be the result of stronger interaction with the zeolite. (ii)One subset may be from clusters within the sodalite cages, while theother is from clusters within the supercages of the zeolite.Interactions between the Ag or AgI moiety and the different cage typesof the zeolite could result in two distinct subsets. (iii) There couldbe two different sizes or conformations of clusters within the cages,for instance, one with a smaller number of atoms (molecules), and onewith a larger number. (iv) One subset may have an extremely efficientnon-radiative decay pathway available relative to the other species.Coupling to trap states in the zeolite or trapping of the excitationswithin the nanoparticle could account for the fast decay. The slowerdecay would then be from a subset of particles which have eitherdifferent configurations or environments such that efficient trappingdoes not occur.

In order to sort out such possibilities, it must be noted that the sizeof the particles is consistent with formation only within the smallersodalite cages, no evidence of larger nanoclusters residing in thesupercages is present. Therefore, it is unlikely that (ii) is correct.Matrix isolated Ag clusters are known to have different absorption andemission spectra depending on size and conformation. If different sizesor conformations produced the two distinct subsets, it would be expectedto find that some PL emission wavelengths would have all or mostly thefast decay component and other wavelengths mostly the slow component.Both components exist at all emission wavelengths, indicating thatselective emission based on cluster size or conformation is unlikely tobe the causative factor. This casts doubt on explanation (iii). Previousoptical measurements of Ag clusters in zeolites have noted that thespectra are similar to Ag clusters in rare-gas matrices. Theseinteractions are considered weak and therefore it is unlikely that suchinteractions would lead to a drastic change in the electronic transitionmoment. Therefore, explanation (i) may be ruled out as well. The onlyconclusion left is that the fast time decay arises from a subset ofspecies that couple to efficient trap sites, either within thenanocluster themselves, or within the zeolite framework. Short excitedstate lifetimes have been observed in AgI colloidal nanoparticles byfemtosecond transient absorption. These short lifetimes have beenrelated to trapping and non-radiative electron-hole recombination withinthe nanoparticles.

Trapping to states within the zeolite serves to shorten the observedlifetimes. Efficient electron trapping in the zeolite must occur inorder to observe the PSL. In addition, trapping occurs at other sitesboth within the zeolite or the nanoclusters. A model that explains thenature of these trap sites and ultimately provides a reasonableexplanation of the observed photophysics is set forth herein (FIG. 10).In order to understand the observed results, consideration must be givento the likely geometrical and chemical distribution of both Ag and AgIclusters within the zeolite. After ion-exchange, Ag⁺ cations areencapsulated into the zeolite cages. In the formation of AgInanoclusters, reaction with I⁻ anions produces AgI nanoclusters in thezeolite cages, while there are still some Ag⁺ ions that coexist alongwith the AgI clusters. In either case, the Ag⁺ ions are ‘autoreduced’ toAg⁰ when the samples are heated in vacuum according to the followingreaction:4Ag⁺+2ZO²⁻→O₂+4Ag⁰+2Z,where ZO²⁻ represents a zeolite framework, and Z represents a zeoliteframework with a missing oxygen link (oxygen vacancy), i.e. with a Lewisacid site. The autoreduced Ag is most likely in close proximity andcoupled to the Lewis acid site forming an Ag:zeolite complex at theinterface.

The near-UV photons used herein are not capable of generating freeelectrons and holes in the zeolite framework. Hence, Ag⁰ is ionized toAg⁺ by UV irradiation (Ag⁰→Ag⁺) leading to both a source of electronsand a luminescent center, Ag⁺. The ionized electron is captured in Lewisacid sites (oxygen vacancies) that are the acceptors of electrons and/orin trap states of AgI or Ag nanoclusters. These electron centers arelikely responsible for the electron spin resonance (ESR) signal. Thetrapped electrons in the zeolite Lewis acid sites produce the F-centernear infrared absorption required to stimulate PSL. Upon furtherirradiation with low energy photons, these electrons are released andmay recombine with the Ag⁺ centers through either tunneling or throughthe conduction band. The photo-released electron may also recombine withAg⁺ to give the emission of Ag⁰ as follows:e+Ag⁺→Ag⁰*→Ag⁰+hλ,where e represents electrons released from the color center, Ag⁰* is theexcited state of Ag cluster, and Ag⁰ is the ground electronic stateafter emitting the photons (hλ). As a result, in either AgI/Y or Ag/Yparticles, only PSL from Ag particles is observed. Thus the PSLoriginates from the Ag:zeolite complex at the interface of thenanoparticles and the zeolite. Likewise, recent results of PSL fromBaFBr:Eu²⁺ phosphors indicate that the PSL originates at interfacesdefined by grain boundaries and dislocations. The sub-nanosecond PLlifetime component also originates from this complex.

Not all Ag clusters will reside near a Lewis acid site in the zeolite,however, nor is it likely that all AgI/Y particles have Ag clustersassociated with them. Those clusters that do not have access to thisefficient electron trap will upon irradiation, display the longer(microsecond) lifetime decays similar to that reported for AgBrnanoparticles. However, those clusters of either Ag or AgI particlesthat have an Ag particle in close association with a Lewis acid sitewill have an extremely short lifetime, and become the luminescent centerupon photostimulation. In addition, there may exist other trap siteswithin the zeolite and migration may occur between them as has beenpreviously described. The existence of additional trap sites andtransfer between them only serves to shorten the lifetime further. Thismechanism is similar to one described to explain PSL in a number ofx-ray storage materials. In the instant application, PSL complexes areformed which have the active luminescent center and electron trap inclose proximity. Tunneling serves to connect the two species. Herein,the Ag⁺-Z site serves as a PSL complex where excitation of the electronfrom its trap within the zeolite results in transfer to the Ag⁺ center.

In addition to the fast and slow regimes evident in the lifetime data,there is significant lifetime decay dispersion with respect to emissionwavelength in both the PL and PSL. In both cases, red emissionwavelengths have slower lifetime decays than the blue wavelengths. Thiscan be explained by noting that small Ag clusters have differentabsorption and emission wavelengths depending on size and geometry.Sodalite cages in zeolite-Y are on the order of 1.3 nm, therefore onlysmall Ag and AgI clusters fit inside. Therefore, a distribution of smallcluster sizes or conformations within the cages is likely. Suchinhomogeneity results in multi-exponential decay lifetimes as well asthe dispersion of the lifetimes with wavelength. Thus, red emissionwavelengths probe different sizes or conformations of the clustersrelative to others. The fact that the PSL spectra from both particles isred-shifted from the PL is a result of certain cluster sizes orconformations being more efficiently coupled to the zeolite oxygenvacancies.

PSL from AgI/Y particles is qualitatively stronger than PSL from theAg/Y particles. The PSL lifetimes are also shorter in AgI/Y compared toAg/Y. Thus, AgI/Y may be a better material for x-ray storage relative toAg/Y. However, short lifetimes and strong PSL make both of thesematerials good candidates for phosphor screens because of thesignificantly enhanced readout rates possible. These materials are alsouseful for digital storage applications as described hereinafter indetail.

Strong PL and PSL are observed from Ag and AgI nanoclusters formed inzeolite-Y. The PL lifetimes demonstrate the existence of two subsets ofnanoparticles, one with a long (microsecond) lifetime and the other witha much shorter, sub-nanosecond lifetime. In contrast, PSL lifetimes showonly a fast, picosecond lifetime. The origin of the photostimulatedluminescence is ascribed to the formation of a PSL complex betweeninterfacial Ag⁺ and Lewis acid sites in the zeolite in close proximity,while the PL originates from both the PSL complex and from nanoparticleswhich are not strongly coupled to the zeolite oxygen vacancies. Thestrong photostimulated luminescence with short decay lifetimedemonstrates that nanoparticles are useful as digital storage mediums,sensors, and medical radiology.

The photostimulable phosphor used in commercial X-ray imaging system isBaFBr:Eu²⁺. Yet, the BaFBr:Eu²⁺ system suffers from poor resolution dueto the long decay lifetime of Eu²⁺ emission (0.8 μs) as well asscattering of the stimulating laser light from the plate-likepolycrystals. Quantum size confinement of nanoparticles overcomes someof these problems. The higher luminescence efficiency, faster decay andlower light-scattering for smaller particles results in higher spatialresolution and faster response. Strong PSL from BaFBr:Eu²⁺ nanoparticleswith a decay lifetime of 390 ns provides a good example demonstratingthat the PSL imaging performance and resolution is improved via quantumsize confinement.

Nanosized BaFBr:Eu²⁺ phosphors were made and a strong PSL is observedfrom these BaFBr:Eu²⁺ nanoparticles. These BaFBr:Eu²⁺ nanoparticles weremade as follows.

The preparation of the BaFBr:Eu²⁺ nanoparticles was based on thefollowing reaction in acid solutions:BaBr₂+NH₄F→BaFBr+NH₄Br

In order to control the reaction rate, stabilizers (e.g., poly(vinylalcohol)) were added to the solution and the reaction temperature wascontrolled. The following methodology was used to make the BaFBr:Eunanoparticles:

A four-neck flask was filled with 400 mL deionized water and 4 gpoly(vinyl alcohol) and was stirred under N₂ for 1 hour. The pH valuewas adjusted to 2 by the addition of nitric acid.

4.83 g BaBr₂.2H₂O and 0.137 g EuI₂ were added to the solution andstirred under N₂ for 1 hour at room temperature.

0.70 g NaF was added to the solution and stirred under N₂ for 0.5 hoursat room temperature until precipitation occurred.

The temperature was raised to 80° C., the reaction was sustained for 2hours and then, cooled rapidly to room temperature.

The nanoparticles were separated from solution by centrifugation, washedwith dionized water, and dried in vacuum at room temperature.

The powder was heated at a temperature lower than 450° C. in carbonmonoxide or in an N₂ atmosphere for 0.5 hour.

FIG. 11 shows the excitation and emission spectra of BaFBr:Eu²⁺nanoparticles having an average size of ˜40 nm, as measured by X-raydiffraction. The emission band is due to the transition of4f⁶5d¹(t_(2g))→4f⁷ of Eu²⁺. It is found that in BaFBr:Eu²⁺nanoparticles, the emission band of Eu²⁺ is at 407 nm, which is ˜17 nmred-shifted compared to that in bulk BaFBr:Eu²⁺ at 390 nm. This is dueto the change in crystal field or site symmetry of the nanoparticles.Furthermore, it is found that in bulk BaFBr:Eu²⁺, only one excitationpeak is observed, while in BaFBr:Eu²⁺ nanoparticles, four peaks appearin the excitation spectra.

The fluorescence of BaFBr:Eu²⁺ nanoparticles is very sensitive to light.Under UV irradiation, the luminescence of Eu²⁺ decreases in intensity asshown in FIG. 12. The luminescence recovers by exposure to red light atapproximately 600 nm. Strong PSL is observed by stimulation at 600 nm,which is shown in FIG. 13. The PSL spectrum is consistent with the PLspectrum. This indicates that the PSL is from the recombination ofelectrons with Eu²⁺ centers as in bulk BaFBr:Eu²⁺ phosphors.

FIG. 14 shows the PL and PSL decays of BaFBr: Eu²⁺ nanoparticles incomparison with that of bulk BaFBr:Eu²⁺ phosphor. The resultsdemonstrate that the PL decay lifetime of BaFBr:Eu²⁺ nanoparticles isalmost the same as that of bulk BaFBr:Eu²⁺, around 800 ns. However, thePSL decay lifetime of BaFBr:Eu²⁺ nanoparticles is much shorter than thatof bulk BaFBr:Eu²⁺. The PSL decay of BaFBr:Eu²⁺ nanoparticles has twocomponents, one is shorter than 20 ns, the other is 340 ns. The shortdecay of less than 20 ns appears to be due to the surface states and thedecay of 340 ns appears to be due to the quantum confinement. Thedifference in the PSL decay behavior of the nanoparticles and the bulkmaterial indicates that the PSL processes in the bulk and thenanoparticles is different. The faster decay in the nanoparticlesdemonstrates that it is possible to improve the storage and imagingperformance by quantum size confinement of nanotechnology.

Undoped nanoparticles also exhibit PSL. In undoped nanoparticles, PSL iscaused by the stimulation of electrons or holes trapped at surfacestates, defects or in the matrices or ths stabilizers. CdTenanoparticles were also prepared and the PSL studied. Strongphotostimulated luminescence was observed in CdTe nanoparticles. FIG. 15shows the absorption, photoluminescence (PL) and PSL spectra of 2.5 and5 nm sized CdTe nanoparticles, respectively. The PL emission maximumwavelength is excitation wavelength dependent, shifting to longerwavelengths for shorter excitation wavelengths, while PSL emissionposition varies little for different excitation wavelengths. The PSLemission band is red-shifted if compared with the Stokes emission, andthe shift is larger for smaller particles. The PSL emission bandwidth isnarrower than that of the Stokes emission band. The PSL is quenched whenthe nanoparticles are oxidized or by surface modification, indicatingthat the PSL of CdTe nanoparticles is related to the surface states.

Described hereinafter is a reversible optical process as observed inEu³⁺ doped nanoparticles. Under UV irradiation, the luminescence of Eu³⁺decreases in intensity and by IR or visible stimulation the luminescencequenching can be recovered. A process involving the conversion of Eu³⁺to Eu²⁺ under UV irradiation and an opposite change from Eu²⁺ to Eu³⁺under visible or IR stimulation is given to explain this newly observedreversible process of Eu³⁺ doped nanoparticles. The followingexperimental results show that Eu³⁺ doped nanoparticles can be used as anew medium for reversible optical storage.

As mentioned previously, in recent years, a demand for improved datastorage has accompanied the dramatic advances in computing andcommunication technology. The requirement for high capacity opticalmemory has led to much research in optical materials, medium structures,and system concepts. Using nanoparticles or quantum dots (QDs) as astorage medium offers tremendous potential. The shortcomings of priorknown approaches using nanoparticles are that they are limited to shortdurations (several hundred microseconds to several seconds) and onlyfunction at low temperature.

As mentioned previously, the photosensitivity of CdSe nanoparticles,silver oxide nanoparticles, and Ag-doped phosphates leads to their usein storage systems. Hereinafter a new way of optical storage apparatusand methodology is given based on the reversible optical processes ofdoped nanoparticles, such as Eu³⁺ doped nanoparticles, at roomtemperature.

Eu³⁺-doped BaFBr nanoparticles were prepared according the following thereaction in water solution.Ba(NO₃)₂+NaBr+NaF→BaFBr+2NaNO₃.

Poly(vinyl-alcohol) was used as a stabilizer. In a four-neck flask,poly(vinyl alcohol) (3 g) was mixed with 200 mL dionized water.Calculated amounts of Ba(NO₃)₂ and Eu(NO₃)₃ (molecular ratio ofEu/Ba=0.5/99.5) were filled into the flask and stirred under N₂ for 30min. Then, calculated NaBr and NaF (molecular ratio of 1:1) were filledslowly into the solution through two necks. The reaction was carried outin a N₂ atmosphere at 80° C. for 3 hours. The nanoparticles wereseparated by centrifugation and dried in a vacuum at room temperature.The formation of BaFBr nanoparticles was demonstrated by X-raydiffraction. The average size of the particles was about 50 nm asestimated from the widths of the X-ray diffraction peaks.

Eu³⁺ clusters were formed into the cavities of zeolite-Y by a well knownmethod called ion exchange, which is expressly incorporated herein inits entirety). The ion-exchange was carried out in water at 80° C.Typically, 2 g zeolite-Y powder and 0.2 g Eu(NO₃)₃.6H₂O filled into aflask with 400 mL H₂O. The exchange reaction was carried out whilestirring in air for 2 hours. Then the Eu³⁺-exchanged zeolite wascollected by filtration, washed extensively with a water-acid (HCl)solution (pH=5), and dried in vacuum at 150° C. for 4 hours. Thephotoluminescence (PL) emission and excitation spectra of thennaoparticles were recorded using a SPEX FLUOROLOG™ (obtainable fromJobin Yvon, Ltd. of the Uniited Kingdom) fluorescence spectrophotometer.Before exchange, fluorescence from the zeolite-Y powder was notobserved. After exchange, strong red luminescence of Eu³⁺ was observedfrom the samples. This indicates that Eu³⁺ ions were encapsulated intothe zeolite cavities by ion-exchange. The optical reversible process wasinvestigated by ultraviolet (UV) irradiation and visible lightstimulation at particular wavelengths from the xenon lamp of thefluorescence spectrometer. All measurements were carried out at roomtemperature.

It was found that the fluorescence of Eu³⁺-doped nanoparticles is verysensitive to light. The emission of Eu³⁺ can be quenched by UVirradiation. Emission is recoverable by visible light stimulation. Thisis a reversible optical process that can be potentially applied foroptical storage.

The emission spectra of BaFBr:Eu³⁺ are displayed in FIG. 16. The fouremission bands are from the transitions of ⁵D₀→⁷F₀, ⁵D₀→⁷F₁, ⁵D_(0→) ⁷F₂and ⁵D₀→⁷F₃, respectively. It is shown in FIG. 16 that under UVirradiation at 225 nm the Eu³⁺ emission decreases in intensity. Thisluminescence quenching is recovered slightly by exposure to a red lightat 600 nm, and it is recovered very quickly by exposure to a visiblelight (500 to 800 nm).

Similar results were also observed in Eu₂O₃ nanoparticles encapsulatedin zeolite-Y, Eu³⁺ nanoclusters in zeolite and Eu³⁺-doped Y₂O₃nanoparticles. One example is shown in FIG. 17 for Eu³⁺ clusters inzeolite-Y. Under UV irradiation at 254 nm, the emission of Eu³⁺decreases. Under stimulation at 840 nm, very little recovery isobserved. However, under stimulation from a visible lamp (500 to 800nm),the fluorescence after UV quenching is recovered almost 100%.

It is noted that the emission spectral patterns of Eu³⁺ in BaFBr:Eu³⁺and Eu³⁺:zeolite-Y are different. In BaFBr:Eu³⁺, the ⁵D₀→⁷F₁, emissionband is more intense than the ⁵D₀→⁷F₂ emission band while inEu³⁺-zeolite-Y, the ⁵D₀→⁷F₂ band is much stronger than the ⁵D₀→⁷F₁emission. This is due to the surrounding environments of Eu³⁺ in the twomaterials being different. The ⁵D₀→⁷F₁ and ⁵D₀→⁷F₂ transitions are theso called “hypersensitive luminescence”, which is highly sensitive tostructure change and environmental effects, and the intensity ratio of⁵D₀→⁷F₁ to ⁵D₀→⁷F₂ provides significant information about the sitesymmetry and structure of the materials.

The physical mechanism for the optical reversible process is thereversible charge transfer between Eu³⁺ and Eu²⁺ ions. Charge transferbetween Eu³⁺ and Eu²⁺ has been previously reported in several systems.It has also been reported that under high energy radiation, rare earth(RE) ions can be oxidized or reduced. This is determined by theoxidation or reduction potentials of the ions. Nugent et al. (1975)extensively investigated the oxidation states of RE ions. According totheir investigations, the oxidation of RE³⁺ to RE⁴⁺ ions is possible ifthe optical electronegativity (χ(RE⁴⁺)) is less than or equal to 3.03,and the standard reduction potential (E_(RE) ⁰(RE⁴⁺→RE³⁺)) is less thanor equal to 5.2 V. For example, the χ(Tb⁴⁺) is 2.55 and the E_(RE)⁰(Tb⁴⁺→Tb³⁺) is 3.30 V, favoring the appearance of Tb⁴⁺ ions. For Eu³⁺ions, it is impossible for Eu⁴⁺ compounds to appear and it is difficultfor the oxidation of Eu³⁺ to Eu⁴⁺, because the X(Eu⁴⁺) (3.40) is largerthan 3.03 and the E_(RE) ⁰(Eu⁴⁺→Eu³⁺) (6.4 V) is larger than 5.2 V. Thisindicates the reduction of Eu³⁺ to Eu²⁺ is more favorable. In addition,Eu²⁺ is a half-filled configuration of 4f⁷. In energy, Eu²⁺is morestable than Eu³⁺. This is another reason for the reduction of Eu³⁺ toEu²⁺ under high energy radiation. Thus, under UV irradiation at 254 nm,electrons and holes are produced in the nanoparticles, and Eu³⁺ions arereduced to Eu²⁺ ions as shown in FIG. 18.

The UV generated holes are trapped in the surface states of thenanoparticles or in the defects of the host materials like zeolites toform hole centers. Under visible or IR stimulation, these holes arereleased from their traps and recombined with Eu²⁺ to form Eu³⁺according to the following formula:Eu²⁺+h→Eu³⁺.Thus, the luminescence of Eu³⁺ can be recovered under visible or IRstimulation. This process is similar to but also different from the PSLof Eu²⁺ in BaFBr:Eu²⁺ phosphors and to hole-burning process in BaFCl:Sm.The similarity is that all the processes involve charge transfer betweenEu²⁺ and Eu³⁺ (or between Sm²⁺ and Sm³⁺). The difference is in both thePSL and hole-burning processes, it is electron centers (F centers)involoved, while in the process described herein, the hole centers areinvolved in the luminescence process. Such a reversible optical materialis highly desirable and particularly useful in optical imaging systems,sensors, and medical imaging syatems, as described hereinafter in moredetail.

As described hereinabove, when Tb³⁺and Eu³⁺ are doped into the samehost, the following reaction occurs:Tb³⁺+Eu³⁺⇄Tb⁴⁺+Eu²⁺.

The above reaction occurs in Y₂O₃:Tb³⁺,Eu³⁺ nanoparticles which can beused for reversible optical storage. The emission spectra ofY₂O₃:Tb³⁺,Eu³⁺ nanoparticles is shown in FIG. 19. Only the emissions ofEu³⁺ are observed from Y₂O₃:Tb³⁺,Eu³⁺ nanoparticles, emision of Tb³⁺ isnot observed. This is due to the efficient energy transfer from Tb³⁺ toEu³⁺ as discussed in the art. After X-ray irradiation for 10 min, theluminescence of Y₂O₃:Tb³⁺,Eu³⁺ nanoparticles is decreased largely asshown in FIG. 19, and an obvious ‘band’ was observed on the sample Thechange in color and the florescence quenching is related to the abovereferenced reaction. The changes in color or fluorescence of thisnanoparticle cannot, however, be recovered by exposure to visible lightor infrared light, and PSL signal is not observed from Y₂O₃:Tb³⁺,Eu³⁺nanoparticles. This indicates that the above reaction induced by X-rayradiation is not reversible. However, the photosensitivity ofY₂O₃:Tb³⁺,Eu³⁺ nanoparticles to X-ray irradiation does provide for otherapplications, such as X-ray lithography.

An additional example of nanoparticles useful for optical storagesystems is ZnS:Mn²⁺, Eu³⁺ co-doped nanoparticles. ZnS:Mn²⁺, Eu³⁺nanoparticles were prepared as following: A four-neck flask was filledwith a solution containing methacrylic acid, citric acid, 500 mLdeionized water, and 500 mL of ethanol. The solution was stirred underN₂ for 2.5 hours. A second aqueous solution containing 8.009 g Na₂S and200 mL of ethanol, and a third aqueous solution containing 3.337 gZn(NO₃)₂.6(H₂O), 0.114 g of Eu(NO₃)₃, 0.2 g of Mn(NO₃)₂ and 200 mL ofethanol (Eu³⁺/Mn²⁺/Zn²⁺ molar ratio 2:3:95) were prepared and added tothe first solution simultaneously via two different necks. After theaddition, the resulting solution was stirred constantly under N₂ at 80°C. for 2.5 h and a transparent colloid of ZnS:Mn²⁺,Eu³⁺ was formed. ThepH value of the final solution was 2.4. This relatively low pH value isrequired to prevent the precipitation of unwanted Mn and Eu species. Thenanoparticles were separated from solution by centrifugation and driedin a vacuum at room temperature.

The X-ray Deflection (XRD) measurement demonstrates that the particleshave the zinc blend structure (sphalerite) (FIG. 20). Broad XRD linesare indicative of small size of ZnS nanoparticles. From theDebye-Scherrer equation the average sizes of the nanoparticles areestimated from half-width of these lines to be ˜3 nm. FIG. 21 shows thehigh resolution transmission microscope (HRTEM) images of the particlesprepared in dispersions. The average size is around 3 nm, which agreeswith the XRD measurements. The (111) lattice planes of some particlescan be observed in the HRTEM images. The (111) lattice spacing of theparticles was estimated to be around 0.31 nm from the HRTEM images. Thisis consistent with the (111) spacing of bulk ZnS (0.312 nm). EDX givesreasonable Zn and S data. About 55-60 atom % Zn and about 40 atom % S.We find 1-2 atom % Eu in all analyses, however, Mn is below thedetection limit (less than 0.2%). These results indicate that sulfur isdeficient in the nanoparticles.

FIG. 22 displays the emission spectra of ZnS:Mn⁺² ,Eu⁺² nanoparticlesexcited at 330 and 344 nm, respectively. The emission peaking at 430 nmis due to the donor-acceptor (DA) pairs which is related to sulfurrelated defects. The emission at 600 nm is attributed to the ⁴T₁ to ⁶A₁transition of Mn²⁺ ions, while the sharp emission line at 615 nm is dueto the f-f transition of Eu³⁺. It is noted that the Eu^(3+e) mission isobserved by excitation at 344 nm but it is not apparent under excitationat 330 nm.

The ZnS:Mn²⁺, Eu³⁺ codoped nanoparticle is sensitive to light. Under UVirradiation at 254 nm, both the emissions of Mn²⁺and Eu³⁺ decrease inintensity. On the contrary, under stimulation at 800 nm, both emissionsof Mn²⁺ and Eu³⁺ can recover in intensity (FIGS. 23 and 24). Thisreversible optical process is controllable by light and is useful foroptical storage methods. Based on our electron spin resonancemeasurements, the mechanism for the reversible process is due to chargetransfer between Mn²⁺ and Eu³⁺. Charge can transfer back and forth fromMn²⁺ to Eu³⁺ as shown in FIG. 25.

The following examples of nanoparticles disclosed and claimed herein maybe summarized into four classes: (1) undoped semiconductor nanoparticles(CdTe, CdSe, ZnO, CdS, ZnS), (2) doped semiconductor nanoparticles(CdS:Mn²⁺, ZnS:Mn²⁺, ZnS:Mn²⁺, Eu³⁺, ZnS:Cu⁺, etc), (3) rare earth (Eu³⁺or Eu²⁺)-doped nanoparticles (Y₂O₃:Eu³⁺, ZnS:Eu³⁺, Zeolite:Eu³⁺,MgS:Eu³⁺, BaFBr:Eu³⁺, BaFBr:Eu²⁺ etc) and (4) Silver, silver halidenanoparticles.

The recipes for making Group II-VI semiconductor nanoparticles disclosedand claimed herein are similar to these reported in literature. Oneexamplary recipe or methodology for making CdTe nanoparticles ishereafter given as follows. The CdTe nanoparticles were prepared by awet chemical technique which has been reported in literature. Cadmiumperchlorate hydrate (Aldrich), aluminum telluride (99.5% pure, Gerac),and thioglycolic (mercaptoacetic) acid (Aldrich) were used as received.CdTe nanoparticles were prepared by the rapid mixing of precursorscontaining cadmium perchlorate hydrate and sodium hydrotelluride(NaHTe), cooled to 5° C., under vigorous stirring. The Cd²⁺ containingsolution was prepared as follows: 0.73 g of Cd(ClO₄)₂.H₂O was dissolvedin 125 mL of water. 0.3 mL of thioglycolic acid (TGA) was added to thesolution and its pH was adjusted to ˜11.2 by the addition of 0.1M NaOH.The solution was then purged with nitrogen for at least 30 minutes. Thesolution of NaHTe was prepared in a vessel cooled with ice water to 5°C., by bubbling an excess of H₂Te through 22 mL of 0.05M NaOH for 40minutes under nitrogen. The hydrogen telluride gas was obtained from thereaction of excessive amounts of Al₂Te₃ and 0.5M H₂SO₄ in an inertatmosphere (nitrogen). Great care was taken to keep the NaHTe solutiontemperature at an average of 5° C., as well as to avoid any contact ofthe solutions involved with oxygen (air) at all times.

After the completion of the reaction, a yellow solution of CdTenanocrystal nuclei was obtained. This solution was then refluxed at 100°C. to promote crystal growth. The size of the particles was controlledby the reaction time. The size of the nanoparticles used in thisinvention is around 4 nm as observed by HRTEM. Most nanoparticle arespherical in shape, while some of them are nonspherical. The (111)lattice strings can be seen from the HRTEM images, and the spacing foundis about 0.36 nm, which is in agreement with the (111) spacing of cubicCdTe of 0.374 nm.

Formation of CdS/BaTiO₃ nanostructured materials: Ba—Ti complex alkoxidesolution and CdS colloidal solution were prepared separately, and mixedtogether to form the composite precuser solution. To make Ba—Ti complexalkoxide solution, barium ethanoxide solution, prepared by dissolvingmetal barium into ethanol, and titanum isopropanide solution in2-methoxidethanol were mixed and stirred for a few hours. CdS colloidalsolution was prepared by a selinazation process of Cd(NO₃)₂ and H₂S fluxin 2-methoethnaol with existence of N,N-dimethlyformamide. The particlesize can be controlled by the concentration of Cd(NO₃)₂ andN,N-dimethlyformamide. The colloid solution as prepared was added to thealkoxide solution with molar ratio of Cd/Ba=0.02, then a clear compositeprecursor solution was obtained. Thin films may be prepared by spincoating with the fresh precursor solution, and then dried at 100° C. inair, heat treated at 200-800° C. in N₂ atmosphere.

The recipe for making uncapped ZnS:Mn nanoparticles is as follows: Afour-neck flask was filled with 400 mL deionized water and was stirredunder N₂ for 2.5 hrs. An aqueous solution of 1.6 g Na₂S and an aqueoussolution of 5.8 g Zn(NO₃)₂.6(H₂O) and 0.26 g Mn(NO₃)₂ (Mn²⁺/Zn²⁺ molarratio 5:95) were prepared and added to the first solution simultaneouslyvia two different necks at the same rate. After the addition, theresulting solution was stirred constantly under N₂ at 80° C. for 24 hrsand a transparent colloid of ZnS:Mn was formed. The pH value of thefinal solution was 2.4. This relatively low pH value is required toprevent the precipitation of unwanted Mn species. The nanoparticles wereseparated from solution by centrifugation and dried in vacuum at roomtemperature. The particle size is around 10 nm as determined byhigh-resolution transmission electron microscope.

The ZnS:Eu²⁺ nanoparticles were prepared in a water-ethanol solution asfollows: In a four-neck flask, the water-ethanol solution (500 mL waterand 500 mL 99.95% ethanol) was stirred while purging with N₂ for 2.5hrs, then a Na₂S solution was added (8.009 g Na₂S dissolved in 100 mL99.95% ethanol and 100 mL deionized water) and a mixed solution of Zn²⁺and Eu²⁺ ( 30.337 g Zn(NO₃)₂.6(H₂O) and 0.114 g EuCl₂ dissolved in amixture of 150 mL 99.95% ethanol and 50 mL deionized water) were addedat the same time with the same speed through two necks, respectively.The reaction was carried out in a N₂ atmosphere at 80° C. for 2.5 hours.Then a transparent colloid of ZnS:Eu²⁺ was then obtained. Thenanoparticles were separated by centrifugation and dried in a vacuum atroom temperature. The pH value of the final solution is 2.4, whichprevents precipitation of other Eu species outside the particles in thesame manner as reported for Mn²⁺-doped ZnS nanoparticles.

The recipe for making ZnS:Eu³⁺ nanoparticles is similar to that forZnS:Eu²⁺ nanoparticles. Just two different points are: it is Eu(NO)₃rather than EuCl₂ that provides Eu³⁺; the reaction is conducted in anair not in a N₂ atmosphere.

The recipe for making ZnS:Ag⁺ nanoparticles is as follows. A four-neckflask is filled with a solution containing 10 mL methacrylic acid and400 mL ethanol (99.95%). The solution is stirred under N₂ for 2.5 hours.A second solution containing 1.6 g of Na₂S and 100 mL of ethanol and athird solution containing 5.8 g of Zn(NO₃)₂.6(H₂O), 0.22 g of AgNO₃, and100 mL of ethanol (Ag⁺/Zn²⁺ molar ratio 5:95) are prepared and added tothe first solution simultaneously via two different necks at the samerate. After this addition, the resulting solution is stirred constantlyunder N₂ at 80° C. for 2.5 hours so that a transparent colloid ofZnS:Ag⁺ is formed. The pH value of the final solution is 3.0. Thisrelatively low pH value is required to prevent the precipitation ofunwanted Ag species from occurring.

The recipe for making Y₂O₃:Tb³⁺, Eu³⁺ nanoparticles is as follows.Solution (A) was prepared by dissolving 15.78 g of Y(NO₃)₃.5H₂O, 0.250 gof Eu(NO₃)₃ and 0.350 g of Tb(NO₃)₃ in ethanol. The solution was stirredfor 2 h at room temperature. Solution (B) was prepared by dissolving0.25 mL of tween-80poly-oxyethylene(20)sorbitate and 0.25 mL ofemulson_OG (oelsauerepolyglyscerinester) in 50 mL of aqueous ammoniumhydroxode solution (pH >10) and stirred for 1 h at room temperature.Solution (A) was then added to solution (B) drop by drop through aburette at a controlled rate (10 drops per minute) with vigorousstirring. The obtained gel was separated in a centrifuge. The aqueoussolution was removed by refluxing in toluene using a water trap. Thetoluene was removed by evaporation, and the resulting powder was driedin the oven at 60° C. for 2.5 h. The final products may be isolated aswhite powders of different sizes after heat treatment in a box furnaceat different temperatures for 3 h in ambient atmosphere.

The preparation of BaFBr:Eu²⁺ nanoparticles is based on the followingreaction in acid solutions:BaBr₂+NH₄F→BaFBr+NH₄Br

To control the reaction rate, we added stabilizers (poly(vinyl alcohol))to the solution and controlled the reaction temperature. In ourexperience, we found the following recipe can successfully make BaFBr:Eunanoparticles:

A four-neck flask was filled with 400 mL deionized water and 4 gpoly(vinyl alcohol) and was stirred under N₂ for 1 hr. The pH value wasadjusted to 2 by nitric acid.

4.83 g BaBr₂.2H₂O and 0.137 g EuI₂ were added to the solution andstirred under N₂ for 1 hr at room temperature.

0.70 g NaF was added to the solution and stirred under N₂ for 0.5 hr atroom temperature until precipitation occurred.

The temperature was raised to 80° C., the reaction was sustained for 2hrs. and then, cooled rapidly to room temperature.

The nanoparticles were separated from solution by centrifugation, washedwith deionized water, and dried under a vacuum at room temperature.

The powder was heated at a temperature lower than 450° C. in a carbonmonoxide or N₂ atmosphere for 0.5 hr.

For making BaFBr:Eu²⁺ nanoparticles in MCM-41, bulk BaFBr:Eu²⁺ powderwas made by solid state diffusion at 800° C. for 2 hours. Then,BaFBr:Eu²⁺ powder and MCM-41 powder (ratio of BaFBr:Eu²⁺/MCM-41 is 5:95)were mixed up and heated at 600° C. under N₂ for 2 hours. The recipe forBaFBr:Eu³⁺ is the same but reaction was conducted in an air rather thanin a N₂ atmosphere.

Referring now to FIG. 26, shown therein is a digital imaging system 10constructed in accordance with the present invention. In general, thedigital imaging system 10 converts energy patterns, such as X-ray energypatterns into digital signals. The digital imaging system 10 can be usedfor medical imaging and diagnosis, as well as radiation detection.

The digital imaging system 10 is provided with an image plate 12, alight source 14, an image reader 16, and an image processor 18. Thelight source 14 emits light generally in the direction of the imageplate 12. The light emitted by the light source 14 is typically of aconstant or known energy level. The light passes through a portion of asubject 20, such as an individual, and is received by the image plate12. As is well known in the art, as the light passes through the subject20 part of the light is absorbed. Thus, the light exiting the subjecthas an energy pattern based on the characteristics of the subject 20.

Referring now to FIG. 27, in general, the image plate 12 includes amatrix 24 and a nanoparticle array 26 operatively associated with thematrix 24. The nanoparticle array 26 is formed of photostimulatedluminescence (PSL) nanoparticles which cooperate to store energyindicative of the energy patterns when the photostimulated luminescencenanoparticles are exposed to such energy patterns.

Referring once again to FIG. 26, in general, photostimulatedluminescence involves storing part of the absorbed energy patterns astrapped electrons and holes in the nanoparticles of the image plate 12.The stored energy is released by the image reader 16. For example, theimage reader 16 can include a laser 34, such as a focused He—Ne laser,which empties the traps by optical stimulation to give luminescence dueto electron-hole recombination. The laser can be scanned by a spinninghorizontal line scanning mirror 36, in combination with a vertical tilttable 38. The photostimulated luminescence intensity, is measured foreach area on the imaging plate 12, using a photodetector 40, such as aphotomultiplier tube or a charge coupled device (CCD), and convertedinto data to be stored in the image processor 18. A fixed mirror 41 canbe provided to keep the remainder of the image reader 16 out of thelight path.

The image plate 12 can be placed in a cassette or holder, for example,and used with conventional X-ray machines, and the digital imagingradiographs can be shared among medical personnel over the Internet oron digital storage media. In addition, large area image plates 12 can beconveniently produced, are reusable, have linear response over a widerange of X-ray or other energy level dosages, and can be erased simplyby exposure to a uniform stimulating light or by thermal clearance.Further, the image plate 12 makes it possible to quantify the positionand intensity of the radiation image. As background, the majorconsiderations for PSL imaging phosphors are listed in Table 1. Theenergy structure in the nanoparticles is important to the effectiveoperation of the detector. First, the emission spectrum should match thepeak sensitivity of commercially available image readers 16. Forexample, the trap depth to the conduction band should be small enough sothat stimulation with commercially available red laser light ispossible, yet sufficiently large to prevent random thermal release ofthe electron from the trap. Also, the stimulating laser light by itselfshould not induce PSL centers, which would result in the nanoparticlesexhibiting high background noise during readout. The nanoparticles musthave a high storage capacity for electrons and holes for goodpost-irradiation stability at room temperature. Generally, the trapdepth should be larger than 0.5 eV to prevent thermal release or fadingat room temperature. The oscillator strength of the absorption of thetrap that is stimulated as well as the recombination probability must behigh in order to reduce the laser power needed to release the storedenergy. High luminescence efficiency is necessary to minimize therequired light, such as X-ray exposures. The decay time of thenanoparticles must be short and the shorter the better. A long decaytime leads to long readout times. Dose linearity and reusabilityfollowing readout or erasure are also important. In addition, thewavelength separation between the stimulation light for reading(corresponding to the color center absorption band) and the monitoredemission light of the recombination center should be sufficiently largeto avoid noise signals due to probing light reflection. Ideally, thenanoparticles should have only one type of trap to prevent signal lossdue to electron migration among different traps.

As posited out that the photostimulable phosphor used in commercialX-ray imaging systems is BaFBr:Eu²⁺, which meets most of theconsiderations reasonably well. The wavelength of BaFBr:Eu²⁺ PSL (˜390nm) is separated from that of the stimulating light (632.8 nm), allowingit to be collected by a conventional high quantum efficiencyphotomultiplier tube (PMT). Compared to other integrating detectors,such as film and X-ray television cameras, the dynamic range of theimaging plate system is much wider. The response of the PSL is linear inthe range from eight X-ray photons per pixel to 4×10 photons per pixel(1:5×10), with an error rate of less than five percent. Finally, theaccumulated background can be erased before use.

However, bulk BaFBr:Eu²⁺ phosphors suffer from poor resolution due tothe long decay lifetime of Eu²⁺ emission (0.8 μs) and excessivescattering of the stimulating laser light. Because the traps are locatedthroughout the depth of the phosphor material, the laser beam providingthe stimulating light must penetrate into the phosphor. Scattering ofthe light within the phosphor causes the release of traps over a greaterarea of the image than the size of the incident laser beam. This resultsin a loss of spatial resolution, which is aggravated if the conventionalimage plate is made thicker to increase the efficiency, making thetechnique less suitable for fields such as mammography where highresolution is necessary. The hygroscopic nature of the conventionalBaFBr:Eu²⁺ phosphor limits the stability of the system. Further, at verylow dosages, such as for X-ray irradiation less than 1 minute,approximating a PSL dose linearity is not a good approximation. Also,electron migration among different color centers reduces the sensitivityand reusability. The same is true for the application of PSL imagingplates for radiation detection.

Significantly, most of the prior art shortcomings can be overcome byquantum size confinement. For example, light scattering and absorptionare both reduced in nano-sized phosphors due to the decreased surfaceareas.

Conventional BaFBr:Eu²⁺ imaging plates suffer from low resolution due tothe scattering of both the reading laser light and the emission light.This scattering is due to the phosphor grain size, shape, and boundaryas well as surface roughness. Rayleigh's approximation for theinteraction of light with matter tells us that for isotropic andseparated particles:

Intensity of Scattered Light≈(Particle Diameter)⁶

This means that a 50 nm particle will scatter one million times morelight than a 5 nm particle. Therefore, compared to traditionalmicrometer-sized phosphors, the light scattering from nanoparticles usedin the formation of the image plate 12 (FIG. 27) is essentially zero.For purposes of explanation, nanoparticles are defined as particleshaving a particle size from about less than 100 nm.

In addition to the particle size, the grain shape of the phosphors isalso an important factor determining the imaging resolution anduniformity. The low resolution with high light scattering of BaFBr:Eu²⁺system is in one way related to this shape. The grains of BaFBr:Eu²⁺ areplate-shaped, which is determined by to the tetragonal space groupP4/nmm. In order to have the nanoparticles in a spherical shape, we needto slow the growth rate in the z-direction and speed growth in the x-and y-directions. Recently, Peng et al. developed a new method forcontrolling particle shapes. The method set forth in Peng et al. (Peng,Z. A. et al., “Formulation of High-Quality CdTe, CdSe, and CdSNanocrystals using CdO as Precursor”, J. Of the American ChemicalSociety, vol. 123, iss. 1, pp. 183-184 (10 January 2001) can be used tocontrol the shape of the nanoparticles. The method set forth in Peng etal. is hereby incorporated herein by reference. Once the particle size,size-distribution, and shape are controlled, the grain boundary and thethin film surface are also controlled.

Referring, once again, to FIG. 27, the nanoparticles can be applied tothe matrix 24 by any suitable method. For example, the nanoparticles canbe applied as a thin film to an exterior surface 44 of the matrix, orcan be doped or otherwise inserted into the matrix 24. The followingmethods are examples of suitable ways for applying the nanoparticles tothe matrix 24: Layer-by-layer assembly: Spin-coating or spin-casting;Spray; Thermal spray; Droping; Supercritical Fluids; Thermal plasmadeposition and Laser ablation.

The matrix 24 can be any material capable of receiving the nanoparticleswithout adversely effecting the photoluminescent properties of thenanoparticles. For example, the matrix 24 can be glass, polystyrene,polymer film or a non-luminescent material. The matrix 24 can beprovided with any suitable 2-dimensional or 3-dimensional shape, such asflat, concave, convex, rectangular, circular, spherical and combinationsthereof. For example, the matrix 24 can be provided with a flatrectangular shape. The matrix 24 can be a flexible or a non-flexiblematerial.

Layer-by-layer (LBL) assembly is the preferred method of applying thenanoparticles to the matrix 24. For example, the advantages oflayer-by-layer thin films are:

-   -   Pinhole-free coatings, which may vary in thickness from        nanometers to millimeters.    -   Optimization of charge-transfer properties of the light-emitting        layer via organized multilayer assemblies.    -   Ordered multicomponent structures, which can be deposited on        traditional solid flat substrates, flexible plastic skins, and        curved sophisticated surfaces such as helmet visors with equal        efficiency.    -   Universality of the deposition-that is, different kinds of        nanoparticles and conductive polyelectrolytes can be used with        minimal variations in the deposition technique.

The driving force for LBL is the electrostatic attraction of positiveand negative charges located on the surface of inorganic colloids andpolyelectrolytes. An important thermodynamic contribution to the filmstability is also made by the van Der Waals interactions. Typically, theassembly process consists of a cyclic repetition of four steps:

-   -   1. immersion of the matrix into an aqueous 0.1-2% (w/v) solution        of a polymer for 1-2 minutes,    -   2. rinsing with ultra-pure water for 30 seconds,    -   3. immersion into an aqueous dispersion of oppositely charged        nanoparticles of PSL material, and    -   4. final rinsing with deionized water for 30 seconds.

Referring, yet again, to FIG. 27, a protective covering 46 can beapplied over the nanoparticles array 26 so as to protect thenanoparticles from abrasion, scratching or other damage. The covering 46is constructed to permit energy to pass through the covering 46. Forexample, the covering 46 can be formed of a plastic film, UV curablefilm or epoxy.

Referring now to FIG. 28, shown therein and designated by a referencenumeral 100 is a schematic illustration of a storage device constructedin accordance with the present invention. The storage device 100 isprovided with a first writing light 102, a second writing light 104, aprobing light 106, a photodetector 108, and an image plate 110 which ispreferably characterized as an optical disk. The image plate 110 issimilar to the-image plate 12, discussed above, except as discussedherein. The image plate 110 includes a particle array 112 associatedwith a matrix 113. The matrix 113 is similar to the matrix 24. Theparticle array 112 is applied to the matrix 113 in a similar manner asthe nanoparticle array 26 is applied to the matrix 24. The particlearray 112 includes a large number of particles or nanoparticles (onlyone of the particles or nanoparticles being indicated by referencenumeral 114 for purposes of clarity). Each of the particles 114 orgroups of the particles 114 in the particle array 112 are capable offorming a separate and independent storage medium for storing digitaldata. Thus, it is envisioned that the particle array 112 will besegregated in a predetermined manner to form a large number of distinctstorage mediums for storing digital data. At least some of the particles114 are switchable from a first state to at least a second state inresponse to exposure of such particles 114 to light of predeterminedwavelengths. The particles 114 have a first emission in the first state,and a second emission in the second state wherein the first and secondemissions are different. For example, the first and second emissions canbe spectral emissions. Examples of substances for forming the particles114 are discussed above, as well as the spectral emissions for suchsubstances.

The term “between” as used herein refers to the switching from a firststate to at least a second state and vice versa. It should be understoodthat some substances forming the particles 114 may be opticallyswitchable between more than two states.

The storage device 100 is also provided with a controller 124. Thecontroller 124 is operably connected to the first writing light 102, thesecond writing light 104, the photodetector 108, and the probing light106 via signal paths 126, 128, 130, and 132. In general, the controller124 controls the first writing light 102 and the second writing light104 e.g. outputs signals, to encode digital information into selectedparticles 114 or groups of particles 114 of the particle array 112. Oncethe digital information is encoded into the particle array 112, thecontroller 124 outputs signals to the probing light 106 and receivessignals from the photodetector 108 to retrieve selected information fromthe particle array 112. The controller 124 or other computer orapparatus forming the storage device 100 is programmed to recognizesignals indicative of the first and second emissions so as to decode thesignals received by the photodetector 108. The controller 124 can beformed of a microprocessor, microcontroller or other logical devicecapable of carrying out or providing the functions discussed herein.Microprocessors and microcontrollers are well known in the art, and thusa detailed discussion of how to make and use microprocessors andmicrocontrollers is not deemed necessary to teach one skilled in the arthow to make and use the controller 124.

As discussed above, the first writing light 102 and the second writinglight 104 selectively emit photons which are capable of being directedto particular particles 114 within the particle array 110 as indicatedby the lines 136 and 138. When one or more of the particles 114 isexposed to the photon emitted by the first writing light 102, theparticle 114 changes from a first state, such as a logic state one to asecond state, such as a logic state zero. When the particle 114 isexposed to the photon 38 emitted by the second writing light 14, thestatus of the particle is changed from the second state to the firststate.

The particles 114 can be formed of any substance capable of beingswitched between at least two states by the exposure to photons. Forexample, the particles 114 can be formed of Eu³⁺-doped substances, suchas ZnS:Eu³⁺; AgI; Ag⁺; Eu²⁺ doped particles such as BaFBr:Eu²⁺; orZnS:Cu⁺, ZnS:Ag⁺. The methods of making such substances are set forthherein above.

As an example, the emission of Eu³⁺ can be quenched by UV irradiationand this quenching can be recovered by visible light stimulation. Thisis a reversible optical process that can be used for optical storage.That is, the first writing light 102 can emit the photons 136 having awave length suitable to form a visible light stimulation. The firstwriting light 102 can be selected so as to provide the photons 136forming visible light and the second writing light 104 can be selectedto provide the photons 38 forming the UV irradiation. In general, thewavelengths of the photons 136 and 138 emitted by the first and secondwriting lights 102 and 104 can vary widely and are determined by theproperties of the particle 114, as discussed above.

The probing light 106 emits photons 140 of a selected wavelength todetermine the state of the particles 114. In one preferred embodiment,the probing light 106 causes the particles 114 to luminesce or otherwiseform the first or second emissions so that the first or second emissionsare received by the photodetector 108. The wavelength of the probinglight 106 is selected to cause the particles 114 to create the first orsecond emissions without causing the switching of the state of thenanoparticles 114. Thus the information stored by the nanoparticles 114is retained. In the example discussed above with respect to theparticles 114 including Eu³⁺, a suitable wavelength for the probinglight 106 is in the Infra red range.

While the first writing light 102, the second writing light 104, and theprobing light 106 have been shown and described separately, it should beunderstood that the first-writing light 102, the second writing light104, and the probing light 106 can either be separate light sources or asingle light source which is tunable to output different wavelengths. Ina similar manner, the path between the first writing light 102, thesecond writing light 104, and the probing light 106 and the particles114 can either be separate paths or the same path.

As will be understood by those skilled in the art, the storage device 10also includes one or more mechanical assemblies (not shown) for movingthe first writing light 102, the second writing light 104, and theprobing light 106 relative to the image plate 110, or for moving theimage plate 110 relative to the first writing light 102, the secondwriting light 104, and the probing light 106 or combinations thereof.These types of mechanical assemblies are typically constructed of tracksand stepper motors and are well known in the art. Thus, a detaileddescription of the mechanical assemblies is not deemed necessary toteach one skilled in the art how to make and use the mechanicalassemblies.

Moreover, it will be understood by those skilled in the art that variouslenses or other optical assemblies, such as masks, can be used to focusor expose only desired portions of the image plate 110 to the photons136, 138, and 140 emitted by the first writing light 102, the secondwriting light 104, and the probing light 106. For example, a mask can beapplied between the image plate 110 and the first writing light 102, thesecond writing light 104, and the probing light 106.

The image plate 102 can be used for various applications where amultiple read-writable storage medium is desired. For example, the imageplate 102 can be used in the formation of floppy or portable disks,CD-ROMS, hard disks, memory, optical imaging and medical diagnostics,radiation detection and infrared sensors.

REFERENCES

The following references, to the extent that they provide exemplaryprocedural or other details supplementary to those set forth herein, arespecifically incorporated herein by reference in their entirety asthough set forth herein particular.

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1. An imaging plate for use in a digital imaging system to store energypatterns, comprising: a matrix; and a nanoparticle array operativelyassociated with the matrix, the nanoparticle array formed ofnanoparticles which cooperate to store energy that is indicative oflatent images when the nanoparticles are exposed to such energy.
 2. Theimaging plate of claim 1, wherein the nanoparticles are capable ofstoring the energy pattern as at least one of trapped electrons andholes.
 3. The imaging plate of claim 2, wherein the nanoparticles arephotostimulated luminescence nanoparticles capable of releasing photonsof a given first energy upon being stimulated with photons of a secondenergy, whereby the photons of the second energy are sufficient totrigger the recombination of the electrons and holes.
 4. The imagingplate of claim 3, wherein the second energy is less than the firstenergy.
 5. The imaging plate of claim 1, wherein the nanoparticles arecapable of switching between a first state and at least one additionalstate.
 6. The imaging plate of claim 1, further comprising a coverassembly positioned substantially adjacent to the nanoparticle array. 7.The imaging plate of claim 6, wherein the cover assembly only allowspassage of predetermined light energies to and from the imaging plate.8. The imaging plate of claim 1, wherein the nanoparticles are dopedinto the matrix.
 9. The imaging plate of claim 1, wherein thenanoparticle array is applied to an exterior surface of the matrix. 10.A method for converting energy patterns into digital signals, comprisingthe steps of: exposing an imaging plate having a nanoparticle arraysupported by a matrix to an energy pattern, the nanoparticle arrayformed of photostimulated luminescence nanoparticles which cooperate tostore energy that is indicative of the energy pattern; opticallystimulating the photostimulated luminescence nanoparticles in the matrixand thereby releasing the stored energy causing photostimulatedluminescence; and converting the photostimulated luminescence into atleast one digital signal indicative of the stored energy.
 11. The methodof claim 10, further comprising the step of measuring thephotostimulated luminescence.
 12. The method of claim 11, wherein thephotostimulated luminescence is measured using a photodetector selectedfrom the group consisting of a photodiode, photomultiplier tube and acharge coupled device.
 13. The method of claim 10, wherein thephotostimulated luminescence nanoparticles are optically stimulated withphotons of sufficient energy to trigger the recombination of trappedelectrons and holes in the nanoparticle array.
 14. The method of claim13, wherein the emitted photons have a greater energy per photon thanthe optically stimulating photons.
 15. The method of claim 10, furthercomprising the steps of: scanning a laser across the imaging platethereby optically stimulating the imaging plate to producephotostimulated luminescence; measuring the photostimulated luminescenceat each area on the imaging plate; and converting the photostimulatedluminescence measurements into a digital image.
 16. The method of claim10, wherein the stated energy patterns are selected from a groupcomprising ultraviolet photons, vacuum ultraviolet photons, X-rays,.alpha.-rays, .beta.-rays, .gamma.-rays or neutrons.
 17. A method forconverting energy patterns into digital signals, comprising the stepsof: exposing an imaging plate having a nanoparticle array supported by amatrix to an energy pattern, the nanoparticle array formed ofnanoparticles which cooperate to store energy that is indicative of theenergy pattern, wherein the nanoparticles are switchable between a firstenergy state and at least one additional energy state where lightemitted from the nanoparticles in the first state is different from thelight emitted from the at least one additional energy state; opticallyprobing the nanoparticles by directing light of a predetermined energyonto at least a portion of the nanoparticle array so as to causenanoparticles exposed to the probe to provide signals; receiving thesignals utilizing a detector; and converting the signals output by thenanoparticles into at least one digital signal indicative of the energypattern.
 18. The method of claim 17, further comprising the steps of:scanning a laser across the imaging plate thereby optically stimulatingthe imaging plate to produce photostimulated luminescence; measuring thephotostimulated luminescence at each area on the imaging plate; andconverting the photostimulated luminescence measurements into a digitalimage.
 19. The method of claim 17, wherein the energy pattern isselected from a group comprising ultraviolet photons, vacuum ultravioletphotons, X-rays, .alpha.-rays, .beta.-rays, gamma.-rays or neutrons.